The invention relates to a servosystem, including adjusting means for changing the position of a mechanical element, a first transducer for forming an adjusting voltage which is a measure of a desired position of the element, a second transducer for forming a response voltage which is a measure of the actual position of the element, and detection means for forming an error signal which is a measure of a difference between the response voltage and the adjusting voltage, which detection means have a first and a second input which are connected to the first and the second transducer, respectively, and an output which is connected to an input of control means for generating, in dependence on the value of the error signal, a control signal for controlling the adjusting means.
A servosystem of this kind, in which the adjusting means are formed by a motor, is known from the abstract in English of JP-A-5-333 901. Systems of this kind are used to adjust the mechanical element to a desired position which is adjusted, for example manually by means of the first transducer. The first and second transducers may include, for example first and second potentiometers, the first potentiometer being adjustable by way of a rotary knob while the second potentiometer is coupled to the mechanical element so that the setting of the second potentiometer changes when the position of the element changes. In systems of this kind a given distance inherently exists between the first and the second transducer and the detection means, so that comparatively long electrical leads are required for interconnection of these parts. A break in one of these leads could cause an uncontrolled and usually undesirable change of position of the mechanical element. In the known servosystem, therefore, steps are taken to ensure that in the case of a break in the lead between the second transducer and the detection means a simulated response voltage is applied to the second input of the detection means. To this end, the adjusting voltage from the first transducer is inverted and applied as an auxiliary voltage, via a resistor, to the second input of the detection means. The value of said resistor is so high that when the connection between the second transducer and the second input is in order, the auxiliary voltage does not influence the detection means. If said connection is interrupted, the auxiliary voltage is applied to the second input of the detection means instead of the response voltage, so that the error signal has the value zero. In that case the motor will not be activated. This wire breakage protection is based on the assumption that the first transducer is fully operational. Therefore, this method is suitable only for counteracting the adverse effects of a wire break in the connection between the second transducer and the second input of the detection means.
A feasible application of a servosystem of the kind set forth is, for example adjustment of the position of the headlights of a vehicle. Using this system, the light beam is directed so that the light lands on the road at a desired distance from the vehicle. In such an application the first transducer may be situated near the instrument panel of the vehicle and the other parts of the system preferably near the headlights. Another possibility consists in that, for example the first transducer includes one or more sensors which measure the position of the vehicle relative to the road, and an electronic circuit which forms the adjusting voltage from the data supplied by the sensors. The sensors may be arranged, for example to measure the height of the vehicle relative to the road near the front and rear axes. The electronic circuit may include, for example a microprocessor. Generally speaking, in such cases a comparatively long and vulnerable connection lead is required between the first transducer and the detection means or between various parts of the first transducer (in said example between the sensors and the electronic circuit), so that the risk of wire breakage in this lead is comparatively high. Therefore, the known servosystem is not very well suitable for applications of this kind.